Soil management for semiarid regions

Agricultural Water Management, 7 (1983) 89--114

89

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

SOIL MANAGEMENT FOR SEMIARID REGIONS

W.E. LARSON', J.B. SWAN 1 and M.J. SHAFFER 2

Department of Soil Science, University of Minnesota, St. Paul, MN 55108 (U.S.A.) 2Agricultural Research Service, U.S. Department of Agriculture, St. Paul, MN 55108 (U.S.A.) Contribution of the Minnesota Agricultural Experiment Station, and the Soil and Water Management Research Unit, North Central Region, USDA-ARS, St. Paul, MN 55108 (Accepted 23 January 1983)

ABSTRACT Larson, W.E., Swan, J.B. and Shaffer, M.J., 1983. Soil management for semiarid regions. Agric. Water Manage., 7: 89---114. Water related climatic factors and soil physical and chemical properties exert a dominant effect on crop production in semiarid regions. Soil management practices which beneficially affect crop production by increasing water storage or through more efficient water use have been intensively studied at a number of locations with their associated soil conditions and under the climatic conditions encountered. The practical benefits of such research have been extensive. The application of field research to specific sites is restricted by the extreme variations in climatic conditions, by the large differences in soil physical and chemical properties, and by the variation in management practices which occur in the region. Because of these soil and climatic differences, the probability levels of expected responses to management practices generally have not been determined. Part of the difficulty has been due to the inability to precisely define and numerically describe the soft and climatic characteristics involved for sites and years other than those in which the research was conducted. Computer-based simulation techniques permit the development of long- and shortterm site specific soft management recommendations with expected results expressed on a probability basis. The research results from specific geographic locations measured under a specific set of climatic conditions can thus be generalized to a much wider range of soils and expressed o n a probability basis using long-term climatic records. Examples are given where crop yield frequencies are computed as a function of longterm climatic conditions involving various management practices. These results illustrate the impacts of a range of residue management, deep tillage, and fallowing practices on crop production in the semiarid environment. The modeling techniques presented can be used with a range of models and potential management practices.

INTRODUCTION Soft management in semiarid regions centers around using the limited water supply efficiently. Water conservation involves maximizing infiltra-

0378-3774/83/$03.00

©1983 Elsevier Science Publishers B.V.

90 tion and storage of precipitation while minimizing evapotranspiration not directly related to crop yield. Techniques used include crop residue management, weed control, snow management, and runoff and evaporation reduction practices (Papendick et al., 1980a). Other considerations include seedbed preparation and planting, residue management, soil fertility, and weed control. Since tillage plays a major role in most water conservation techniques (Compton, 1942), as well as in other soil management considerations, a major portion of this presentation will deal with tillage and tillage-related practices. From the standpoint of the practitioner, tillage has remained as much art as science. Hillel (1972) comments that "for the most part prevalent methods of seedbed preparation and of seeding are still based on the accumulated experience of generations and on trial and error methods of experimentation". This is due to the complexity of the problem and to the limited number of experimental sites and limited range in weather. In contrast, tillage recommendations must apply to a large variety of soil types and highly variable environmental conditions. To be most useful, management recommendations must be crop and site specific. Ideally, they should be evaluated over a 25 to 100 year range of climatic conditions, and expressed on a probability basis. Fruitful research on tillage-soft water-fertility effects and interactions requires integrating what is known into prediction equations or models. Functional relationships should be derived from experimentally determined information at specific sites. These can be built into models capable of extending the relationships to farmers' fields by the use of local soils and weather information. COMPUTER SIMULATIONS The development of computer simulation techniques for soil-crop systems has made possible rapid evaluation of management alternatives in the semiarid environment. Appropriate models have been developed by several research groups (Dutt et al., 1972; Shaffer et al., 1977; Hanks and Hill, 1980; Knisel, 1981; Shaffer and Gupta, 1981; McKinion and Baker, In press; Williams et aL, In press. The nitrogen-tillage residue management (NTRM) model developed by USDA-ARS at the University of Minnesota (Shaffer and Larson, 1982; Shaffer et al., In press) is a comprehensive simulator of the soft-crop-water system in one dimension for the unsaturated zone and two-dimensions below the water table. Emphasis is placed on plant and root growth, tillage, water flow, nitrogen transformations, and crop residue interactions. A mechanistic oriented modeling approach which adequately simulates the interactions of the crop with the physical, chemical, and biological parameters of the system allows extrapolation beyond current data sets. A stochastic approach to model input data and coefficient variability can

91

then be used to generate yield frequency curves and ultimately, confidence bounds for model output. A suitable model for use in testing management alternatives should be based more on model reliability and validity than on computer costs per run. A good model can be used to evaluate management impacts in the long- and short-term without making a large number of computer runs or making individual runs involving many successive years. This evaluation is done by carefully selecting the input data sets, running representative crop sequences, and using efficient methods of sampling statistical distributions. The NTRM model was utilized in this study to examine various soil management practices proposed for the semiarid Great Plains, and to demonstrate some applicable modeling techniques, while generating insight into potential methods for alleviating drought impacts. A transect was taken in a north-south direction from the Dakotas through the Texas-Oklahoma panhandle (Fig. 1). Several methods of soil management were analyzed at three hypothetical sites located near Bismarck, ND (ND), North Platte, NE (NE), and Amarillo, TX (TX). Projected yields were made for grain sorghum and wheat without irrigation. The assumptions were made that the softs were silt loams, adequate crop nutrients were available, and weeds, insects, and diseases were controlled. Simulated site specific, daffy values for precipitation, maximum and minimum air temperatures, and solar radiation data were generated for a 100-year period using the Richardson-Nicks climate generator model (Nicks, 1974; Richardson, 1981; Williams et al., In press). The latter three parameters were used to calculate

Fig. 1. Location of study sites.

92

daily pan evaporation (Campbell, 1977). Long-term annual precipitation distribution was simulated for each site (Fig. 2). Using the 100-year data bases, three sets of representative years were selected, which approximated the values for the mean annual precipitation and two standard deviations on each side of the mean. Final selection of representative years at each site was based on growing season precipitation. The wide data range (Table I) provided definition of the yield response curves suitable for interpolation using a regression approach. 0.3

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Fig. 2. Long-term distribution of precipitation at the study sites.

TABLE I

Stochastic (simulated) precipitation at the three study sites used as model input Location

Precipitation (cm) Wet Annual

Average Growing

Annual

season a

Bismarck, ND North Platte, NE Amarillo, TX

59.9 73.9 82.8

38.9 47.8 55.6

aDefined as 1 April to crop maturity.

Dry Growing

Annual

Growing season

24.4 29.2 20.8

13.5 21.8 12.7

season

40.6 51.5 51.8

28.7 33.3 31.8

93 The computer simulation approach was used to demonstrate effects of continuous wheat vs wheat-fallow, effects of compaction vs non-compaction, and amounts of surface residues on crop yields at the three locations. Additional areas where appropriate models would be of benefit are reviewed and include the effect of: (a) tillage on evaporation; (b) seed placement and seedsoil contact on germination and emergence; (c) snow retention on water storage and yield; and (d) nutrient-water interactions on crop yields. Space limitations prevented demonstrations of the model capabilities in the latter cases. RESIDUE A N D C R O P M A N A G E M E N T

The kinds and amounts of plant residues available are determined by the cropping and management system. Residues of wheat, barley (Hordeum vulgare L.) and oats (Arena satina L.) predominate in semiarid regions with sorghum (Sorghum bicolor L. Moench) or corn (Zea mays L.) residues also present. The amounts of crop residues produced per unit area vary widely between the 29 Major Land Resources Areas (MLRAs) in the Great Plains (Skidmore et al., 1979). Dryland wheat often yields from 1 to 2 t ha -1 year -1 of straw in the southern Great Plains, and from 3.4 to 4.7 t ha -1 year -1 in the Central Great Plains (Greb et al., 1979). The number and type of tillage operations determine the percentage of surface residue remaining after tillage. While a considerable variety of equipment is available, surface residue systems, sweeps or blades are generally operated 12 to 15 cm deep during the initial operation following harvest and shallower during subsequent operations. Uniform spreading of crop residue by the combine is necessary to minimize tillage problems (Unger and McCalla, 1980). When large amounts of residue are present, a disk-type implement may be used in the initial operation to incorporate more of the residue, yet still retain sufficient surface residue for erosion control. Other implements that may be used in heavy residue include skew-treaders or spike-tooth harrows in conjunction with disk plows (Papendick and Miller, 1977). The use and limitations of summer fallow to store water was reviewed by Black et al. (1974), Greb et al. (1974, 1979), Johnson et al. (1974), Johnson and Unger (1976), Smika {1976) and Willis (1976). While fallow usually increases both soil water content at planting and crop yield, grain production in the Southern Great Plains on a total area basis has been lower with fallow systems {using one-way disk or stubble mulch tillage)than with continuous cropping (Unger, 1981). A major factor contributing to limited soil water storage during fallow in southern areas is the small amount of residue produced. When increased wheat residues due to irrigation were retained on the surface during fallow, significant increases in water storage and grain yields of sorghum were measured (Musick et al., 1977; Unger and Wiese, 1979). Thus, there appears to be opportunity for increased total

94 production through judicious selection of cropping and/or irrigation-dryland sequences. To investigate the impacts on wheat yields of fallow versus continuous cropping, simulation runs were made using the NTRM model on each of the three study locations. The regression for straw yields versus precipitation at NE are shown in Fig. 3, and illustrate the technique of interpolation between points generated with the NTRM model. To test those results against experiment, a plot was prepared (Fig.4) which compares these data with similar data reported by Allmaras (1983). Figs. 5, 6, and 7 show the results for the long-term yield frequencies for ND, NE, and TX. The skewed distributions for the case of continuous wheat are replaced with a more normal distribution for wheat-fallow. Thus, one can achieve higher wheat yields and a greater certainty of average yield in any given year with fallow. The cumulative probability of wheat straw yields in ND equal to or greater than those specified are plotted in Fig.8. Both continuous and wheat-fallow management result in similar straw yields under wet annual climatic conditions. The benefits obtained from the wheat-fallow technique increase rapidly under drier conditions with the maximum obtained at about the 30 to 40% probability levels. Under conditions of limited available water, increased early growth has resulted in more rapid water use, which later increased drought stress and

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95 WHEAT FALLOW STUDY

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96 NORTH PLATTE WHEAT-FALLOW STUDY

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Fig. 6. Long-term distribution of straw yields under continuous and wheat-fallow conditions (North Platte, NE).

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Fig. 7. Long-term distribution o f straw yields under continuous and wheat-fallow conditions (Amarillo, TX).

97 BISMARCK WHEAT-FALLOW STUDY

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Fig. 8. Probability of wheat straw yields under continuous and wheat-fallowconditions (Bismarck, ND). decreased grain yields (Viets, 1966; Amemiya). Such observations are in agreement with the greatly increased effect of drought stress on grain yield during the reproductive stages compared to the vegetative stages for grain crops. Stewart and Musick (1982) cite reviews showing strong linear correlations between cumulative seasonal transpiration and drymatter production when yields are transpiration limited. When transpiration and evapotranspiration are closely associated, drymatter production and cumulative evapotranspiration also show a strong linear relationship. For grain yield, the relationship with seasonal evapotranspiration is often linear but with a different slope than with drymatter. However, based on data of Musick and Dusek (1971) for grain sorghum, Stewart and Musick (1982) argue that the water use yield function is not an explicit relationship and that it can vary considerably depending on various factors which affect both yield and water use. They conclude that limited irrigation, particularly during more critical growth periods, shows considerable promise for increasing water use efficiency. The observed curvilinearity at higher evapotranspiration levels offers some opportunities for other mechanisms of increasing water use efficiencies, presumably including the management of drymatter production rates. Use of legume green manure crops for N fixation is restricted by the climatic conditions and the large evapotranspiration amounts (Hobbs, 1957; Rogers and Giddens, 1957; Homer, 1960; Homer et al., 1960; Shrader

98

and Voss, 1980). Corn yield decreases following alfalfa are well d o c u m e n t e d under conditions of insufficient water, a c o m m o n condition in the western cornbelt (Voorhees and Holt, 1969; Sloan, 1976; Voss and Shrader, 1979). WATER MANAGEMENT

Tillage From the literature, Burnett and Hauser (1967) concluded that crop yields were increased most by deep tillage when water was a limiting factor and fertility or salinity was not limiting. The primary benefit of deep tillage was through one or a combination of mechanisms: (a) increased root proliferation which enables the plant to extract water from a larger volume of soil; (b) increased stored soil water due to improved water entry; or (c) increased stored soil water due to a change in particle arrangement. Deep tillage was most effective in increasing crop growth under dry conditions in Canada (Krogman and MacKay, 1980). Deep plowing may assist in reclaiming some salt affected softs (Burnett and Hauser, 1967; Krogman and MacKay, 1980; Karkanis and Cairns, 1981). In portions of the U.S.S.R., "deeper than normal" tillage is extensively used (Papendick et al., 1980a). To test the hypothesis that deep tillage mitigates the effect of drought on crop yields, simulation runs were made on the silt loam soil under compacted conditions and after removal of the compacted zone by a chisel plow. The compacted zone with a bulk density of 1800 kg m -s was assumed to begin at a 15 cm depth and allowed little root penetration. The initial soil water content was assumed to be 0.32 cm s cm -3 (field capacity). Fig. 9 illustrates the straw yield responses for the two cases in ND and TX, respec6-

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Fig. 9. Simulated long-term wheat straw yields at Bismarck, ND and Amarillo, TX.

99 tively. The impact of deep plowing on yields is generally similar for TX and NE (not shown) with a rather pronounced increased benefit shown in ND. Figs. 10 and 11 illustrate the results obtained after applying the stochastic precipitation data to the yield curves in Fig. 9. The mean yields were increased as deeper tillage increased the available water storage in the soil. Deep plowing in ND significantly increased the standard deviation of the mean; although the error is uniformly distributed. The straw yield values in TX are hypothetical since it would be extremely difficult to consistently begin the growing season with a soil water content near field capacity. The yield differences shown between study sites illustrate the importance of weather patterns, planting dates, and their interactions given the same initial set of soil conditions. Surface roughness (Burwell and Larson, 1969) and surface residue cover of tilled soil (Mannering and Meyer, 1961; 1963) can markedly enhance water infiltration. Van Doren and Allmaras (1978) related the change in soil surface saturated conductivity with the sum of the rainfall kinetic energy, a soil structural stability constant, and the fraction of the soil surface covered by crop residue. The lower the initial soil surface saturated conductivity, the less effective crop residues are in recharging the soil water supply. Under a range of tillage conditions, Allmaras (1967) found that, on two soils, water storage was positively correlated with cumulative intake; but on a third soil, the results were reversed. The amounts and frequencies of precipitation interacted with the initial profile water content, roughness, porosity, and length of evaporation period between rainfall events to produce differences in distribution and water storage in the soil profiles. In such circumstances, simulation techniques using long-term meteorological data and appropriate soil-water relationships (Linden, 1982) are necessary to assess the probability of given levels of storage expected under different tillage methods.

Residue management When sufficient amounts were available, surface crop residues have increased water storage and yield (Greb et al., 1979; Smika et al., 1969; Unger, 1978). In the southern Great Plains, Unger (1978) found that 1 or 2 t ha-1 of crop residue produced by winter wheat was insufficient to substantially increase yields over conventional stubble mulch tillage. He suggested that low residue production of dryland crops in this region is a major reason for low storage efficiency of precipitation. Crop residue mulches effectively decrease evaporation during the first stage evaporation (Unger and McCalla, 1980). For maximum water conservation over long periods, enough water must be added to penetrate deeply into the soil profile or large amounts of residue must be present.

100 BISMARCK COMPACTION STUDY

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Fig. 10. Long-term wheat straw yield distribution under compacted and chisel plow conditons (Bismarck, ND).

AMARILLO COMPACTION STUDY

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101

The effect of between growing season residue amounts on grain sorghum yields at three sites and under site specific climate input was predicted using the NTRM model. It was assumed that straw residues were applied at rates of 0, 4, and 8 t ha -1 which resulted in spring (1 April) soil water contents of 0.1, 0.21, and 0.32 cm 3 cm -3, respectively. Residue during the growing season was maintained at a rate of 2 t ha -1. The regression curves in Fig. 12 (TX) were fitted to yield response data calculated by use of the NTRM model. The results provided families of curves which characterize the yield response to precipitation for the residue treatments. The model output was compared with experimental results (data points shown) obtained by Unger (1978) at Bushland, TX which are displayed on the plots for Amarillo, TX. Unger's results have been approximately reproduced by the model illustrating the general agreement of model output with experiment. The curves shown in Fig. 12 (TX) and similar curves for NE (not shown) were used in conjunction with the stochastic precipitation data to produce the yield distribution curves shown in Fig. 13 (TX) and Fig.14 (NE). The distributions represent the responses at each seasonal precipitation value. These figures illustrate the changes in yield distribution, mean yields, and standard deviations for the three assumed residue treatments analyzed over the entire historical record. The increases in the mean annual yields and decreases in the standard deviations of the mean yields for the residue versus the non-residue treatments indicate a definite long-term increase in plant available water. Thus, STRAW RESIDUE MGMT STUDY. TEXAS

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Fig. 12. Predicted and observed sorghum yields at Amarillo, TX.

100

102 RESIDUE STUDY, AMARILLO 0.8 RESIDUE ( t 0 4 MEAN SX sd 06--

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Fig. 13. Long-term yield distribution for sorghum (Amarillo, TX).

RESIDUE STUDY, NORTH PLATTE, NE 0.4 RESIDUE ( t MEAN Sx Sd

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Fig. 14. Long-term yield distribution for sorghum (North Platte, NE).

103 the chances of obtaining an average yield are increased by maintaining residues on the soil surface between growing seasons. Yield levels are slightly reduced under heavy mulch treatments at NE.

Evaporation reduction Hillel and Rawitz (1972) point out that the choices for reducing bare soil evaporation depends on the stage of the process of concern. In first stage evaporation, meteorological and surface conditions dominate; while during the second stage, the rate of water supply to the surface is determined by transmission characteristics of the profile. As a consequence, deep tillage to modify the diffusivity of the profile may have little effect on stage one evaporation, but could affect second stage drying. In contrast, surface mulches retard evaporation during the initial stage of drying, providing plants a greater opportunity to utilize water in surface soil layers which is important during germination and establishment phases. Mulches can also enhance internal drainage to greater depths where it is less likely to be lost by evaporation (Hillel and Rawitz, 1972). Tillage often enhances drying of the tilled layer, but can also reduce water movement from layers below. The net effect of tillage on storage depends on a number of factors including: the duration of the process; the depth, degree, and frequency of tillage; subsequent rainfall (amounts and timing); and the influence of rainfall on reconsolidation (Hillel and Rawits, 1972). A major restriction on water conservation effects from tillage is that by the time the soil surface is dry enough to cultivate, the first stage of evaporation is generally over. Thus, Allmaras's (1967) experimental results illustrate HiUel and Rawitz's (1972) statement that "tillage might thus produce either a larger or smaller loss than would have occurred in the undisturbed soil...". Aggregate size can affect evaporation rate from seedbeds. Holmes et al. (1960) found that in the laboratory the optimum size of aggregate was 2.5 mm for minimizing the rate of evaporation. Beds of larger or smaller aggregates had higher evaporation rates. Hillel and Hadas (1972) recommended aggregates of 0.25 to 2.0 m m diameter on the soil surface for retarding evaporation from bare soft. Additional research on achieving 'two layer' systems with a coarse layer of large pores overlying a finer structured profile is warranted as this appears to offer the best conditions for both evaporation and infiltration control (Hillel and Rawitz, 1972).

Seed placement The basic interactions of physiological and environmental factors affecting seed germination and seedling establishment have received little attention until recently. Soil water, temperature, mechanical strength, aeration, and salinity conditions which prevail at the surface are known to be of

104

importance, but their combined and separate effects are incompletely described by the data available (Hillel, 1972; Prihar, 1982). Prihar indicates that since aeration in seedbeds is generally non-limiting, the major concern in seedling emergence reduces to the management of water, temperature, and soil physical impedance. A n u m b e r of important seedbed factors affecting winter wheat emergence and early growth have been incorporated into an emergence model (Lindstrom et al., 1976). To achieve rapid germination and early growth, seed placement in moist soil is essential, as is prevention of rapid drying of the layers around the seed (Sepaskhah and Ardekani, 1978; Hillel, 1972). Soil water plays a dominant role in root growth because it not only affects the water availability to the root, but also influences the temperature, mechanical strength, and aeration status surrounding the root (Prihar, 1982). Increasing the normal depth of planting 2.5 to 10 cm can have an adverse effect on cereal germination and seedling emergence (Hart, 1964; Gul and Allan, 1976). De Jong and Best {1979) found that increasing the planting depth not only increased the heating requirement for emergence, but also increased the variability of emergence. The type of planting equipment is also important for proper seed placement. Shovel and hoe opener drills are generally more satisfactory for planting small grains through a dry surface soil and placing seeds in moist soil than are drills with disk openers (Unger and McCalla, 1980). Double- or triple-disk press drills often provide insufficient penetration for proper seed placement in untilled surfaces when soil bulk density in the upper 5 cm exceeds 1200 kg m -3, or when residue exceeds 3.7 t ha -1 on the surface (Lindwall and Anderson, 1977). Greb (1979a) credits improved deep furrow planters (12 to 14 cm) with 8 percent of the recent commercial wheat yield increases. Good seed-soil contact is also necessary for rapid germination. Hadas and Russo (1974) r e c o m m e n d e d that seedbeds be prepared so as to ensure high soil water conductivities, and as large a seed-soil water contact area as possible without impairing radicle elongation or aeration conditions. Hadas and Russo (1974) found that seed-soil water contact impedance increased as the wetted soil area and/or soil hydraulic conductivity decreased. They concluded that the mean aggregate size should be one-fifth to onetenth of the seed's diameter for optimal seed-soil water contact. Such a requirement may conflict with Hillel and Hadas (1972) who recommend 0.25 to 2.0 mm diameter aggregates on the soil surface to retard evaporation from bare soil. Consequently, the row and interrow area may require somewhat different tillage. However, root-sol: contact may also be an important factor affecting water uptake, which may restrict the latitude over which aggregate size may be varied. Herkelrath et al. (1977) proposed that as the soil dried, the surface area of roots in contact with the soil decreased, which caused an increase in root membrane resistance. They found that by assum-

105 ing membrane permeability proportional to the relative saturation of the soil, a much better fit to the water uptake data was obtained. Hadas and Russo {1974) conclude that in the sowing row, a good stand can be assured by reducing the aggregate size around the seed or by compacting the soil area. Any treatment which reduces seed-soil water contact impedance should be adopted since a prolonged imbibitional period due to high contact impedance increases the extent of drying of the topsoil. Snow retention

Considerable potential exists for increasing water storage through improved snow catchment in portions of the central and northern Great Plains. In one winter, the soil water accumulation from improved snow retention could be as large as the soil water accumulations from summer fallow {Snyder et al., 1980}. Research from the U.S.A., Canada and the U.S.S.R. has demonstrated gains in soil water from snow catch by stubble management {Papendick et al., 1980a). Management techniques used were: (a} tall stubble; {b) alternating stubble heights; and (c) use of 15-m strips of standing stubble; all three techniques were effective under the conditions encountered. Research cited by Papendick et al., {1980a} demonstrated the effectiveness of tall wheat grass barriers at Sidney, MT and Akron, CO. In Montana tall wheat grass barriers with 14.6 m cropping intervals increased water storage to 9.9 cm in the protected area, as compared with 5.3 cm in adjacent unprotected areas with continuous cropping {Black and Siddoway, 1976). With a crop-fallow system, the barriers increased water storage efficiency only during the first winter {9-month) period. With continuous cropping an average of 71 to 80% o f between-harvest precipitation was used compared to only 30% for spring wheat-fallow. Sunflower, sunflower and mustard, and tree shelterbelts are used to increase snow retention and water storage in various part of the U.S.S.R. In the U.S.S.R., some type of snow retention measures are applied to 60 to 60 million ha year -1, mostly in Kazakhstan and Siberia (CIA, 1974). Snow ridges created by special plows are used in the USSR on a significant acreage {CIA, 1974). Long-term yield increases of 270 kg ha -1 are reported (Papendick et al., 1980a). The practice is most efficient when combined with vegetative windbreaks, alternating stubble heights, or tall stubble because a minimum snow depth of about 10 cm is required to form ridges. A rough soil surface may increase soil water storage because of increased snow catchment and infiltration. Tillage techniques which leave a rough surface often retain a portion of the crop residue on the surface and thus a portion of the benefit is due to the crop residue. In the intermountain dryland region, contour fall chiseling below the frost line reduced runoff and stored 87% of the September--April precipitation compared to only 60% stored by standing stubble (Ramig et al., 1983).

106 CONTROL OF WIND AND WATER EROSION

Water and wind erosion estimates for the ten Great Plains states are given in the RCA reports (USDA, 1981) (Fig. 15). Wind erosion is greatest in Colorado, Texas and New Mexico. Average water erosion amounts are greater than wind erosion amounts in Nebraska, Kansas, Oklahoma and North Dakota. I . . . . . . .o

I

i

IIII

Wind Erosion Water Erosion

Ko.sas Montono

I

~

Ne~os~a ,, !

- - - -Me~Ico Ne*

Oklahomo Sou~hO~ko~o

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I0 15 ( t ha -1 )

20

25

30

35

Fig. 15. Average wind and water erosion rates on cropland in the Great Plains states. In general, tillage practices for wind and water erosion control are similar. Wind erosion control is best achieved through use of any or all four basic management principles: (a) cloddy surface to resist wind force; (b) furrows or ridges perpendicular to the wind direction to slow the wind speed; (c) stripcropping to reduce wind fetch; and (d) adequate vegetative cover to reduce wind speed at the soil surface and to anchor the soil. While wind erosion decreases as the amount of residues maintained on the soil surface increases, the control is more marked once the mulch exceeds 1.1 t ha -1 (Greb, 1979b). Because of weather variability, it is not always possible to produce enough crop residue to protect the soil against wind and water erosion. Allmaras (1983) has given the probabilities of achieving straw yields below a specified maximum value in a wheat-fallow cropping system and in a continuous cropping system of the same wheat type. Of the four regions compared, climatic variability affected straw production most in the central Great Plains winter wheat region. In the southwestern subregion of the central Great Plains, less than 1.21 t ha -1 would be produced in 2 of 10 years; 1.98 t ha -1 in 5 of 10 years; and 5.02 in 8 of 10 years in a wheat-fallow cropping system. In a continuous wheat system, less than 0.41 t ha -1 would be produced 2 of 10 years; 1.65 in 5 of 10 years; and 3.39 in 8 of 10 years. Straw production is usually higher in a wheat-fallow system as compared with a continuous wheat system.

107 In a wheat-summer fallow management system, at least 50% of the straw at crop harvest is either buried or decomposed by the end of the fallow year (Greb et al., 1970). Allmaras (1983) has calculated that if 0.825 t ha -1 of straw remains after wheat seeding, soil loss would be 34% of that from a fallowed bare soil surface exposed to the same rainfall and runoff energy. The same amount of straw after wheat seeding would reduce wind erosion to about 50% of that from a bare and smooth soil surface. Thus, if it is assumed that 50% of the straw is lost by a conservation tillage system, at least 1.65 t ha -1 would need to be produced. Management practices that have flexibility to meet the extreme variation in weather and resultant crop production are needed. For example, in years when crop residue production is small, herbicides for weed control, and seeding equipment which conserves the residue on the soil surface might be used. In years of abundant residue availability, a combination of tillage and herbicides might be used. Emergency tillage to reduce erosion is sometimes necessary when amounts of residue are insufficient or when other management practices have been inadequate. Emergency tillage at right angles to the slope direction is most effective for water erosion control. Emergency tillage for wind erosion control should provide soil roughness by bringing clods to the surface or by construction of ridges and furrows, preferably perpendicular to the prevailing wind direction. Vegetative barriers grown in rows break the force of the wind. Barriers may be grown with the double objective to trap snow and to protect against wind (Aase et al., 1976). Desirable barrier characteristics include: (a) narrow double rows to insure against wind gaps; (b) plants 0.4 to 1.2 m tall with flexible stalks to minimize breakage; and (c) stalk populations to provide 65 to 75% air porosity (Greb, 1979b). Barriers have been grown in strips 9 m to 18 m apart (Black and Siddoway, 1971). Tall wheatgrass (a perennial), sorghum, and sudan grass have desirable characteristics for barriers. Disadvantages of using vegetative barriers include establishment and maintenance costs, inconvenience in farming operations, and water use by the barrier. Windbreaks of shrubs or trees are often used for reducing wind velocities at the soil surface and to lower erosion and increase snow trapping. The usual recommendation of spacing for field windbreaks is about 10 times the height of the trees. NUTRIENT MANAGEMENT Return of crop residues as opposed to their removal is an important factor affecting the recycling of plant nutrients (Papendick et al., 1980b). Nitrogen and P are the major nutrients of concern in Great Plains crop production, y e t most of the N and P taken up by the plant is removed in the grain. For wheat, the N and P in crop residues represent only 19 and

108

15%, respectively, of the N and P in the grain portion (Holt, 1979). Returning crop residues returns most of the K. Animal manure adds N and P to the soil and can increase the availability and uptake of soil and fertilizer P by plants in alkaline softs (Olsen and Flowerday, 1971). On alkaline soils, organic matter has increased the concentration of P in the soil solution (Olsen and Barber, 1977). Because the degree of soil disturbance generally affects mineralization rates of N and P from soil organic matter (Russell, 1961) introduction of reduced or 'no tillage' techniques may slow the release rates of N and P. For example, at Bushland, TX, one-way disk tillage resulted in a more rapid loss of organic matter and higher soil nitrate content than stubble mulch tillage. A wheat-fallow system resulted in a more rapid loss of organic matter and higher soil nitrate content than did continuous wheat (Johnson et al., 1974). Finally, tillage methods that retain residue on the surface result in slow decomposition of the residues and release of nutrients (Unger and McCalla, 1980). Fertility-water interactions for dryland crops have been well documented (Viets, 1962; Bauer et al., 1965; C.A. Black, 1966). The effects is clearly demonstrated in data of Ramig and Rhoades (1963) and cited by Black (1966) for winter wheat in western Nebraska (Fig. 16). Yield increased as N fertilizer increased for the treatment with 20.6 cm of available water at planting. However, yields were not greatly different at all N fertilizer levels for the 0 cm available water treatment. Black (1966) observed that for grain crops grown "with a small supply of available water, it may be desirable to limit the rate of exhaustion of water by limiting the growth of vegetative parts of plants, thereby saving a greater proportion of the available water for the stage of grain development".

~3

~ l J

evoilable

! O[

0

r --

avadQble i

22 FERTILfZER

i

45 N APPLIED

i

90 PER HECTARE (k 9)

Fig. 16. Yield of grain and total dry matter of winter wheat in western Nebraska with different applications of fertilize~ nitrogen and different quantities of available water. The figures in the graphs correspond to the centimeters of available water in the soil at planting time.

109

Regulating the time of N uptake by date of application appears to be a useful tool for maximizing the efficiency of use of N fertilizer and water. The interaction of N fertilization and soil water with yield under dryland conditions is well d o c u m e n t e d (Bauer et al., 1965; Young et al., 1967; Black and Siddoway, 1976). Young et al. (1967) developed equations relating yield of spring wheat and barley to stored available soil water and NO3--N at seeding, and precipitation and temperature. Smika and Grabouski (1976) concluded that both soil NO3--N and soil water had to be at similar depths for the NO3--N to effectively increase grain protein. In an aridic Haplustoll, application over 1 year before seeding gave maximum accumulation of nitrate in the soil between 60 and 120 cm, and produced the greatest yield and highest protein. In a Torripsamment, NH3 application 2.5 to 4 months before seeding produced the highest protein grain and highest grain yields. Deep placement significantly increased yields of wheat and grain protein content in field studies in the state of Washington (Cochran et al., 1978). Black and Siddoway (1976) studied the effects of three levels of N fertilization (0, 23, and 67 kg ha-') on the yield of different crops grown under various crop sequences. They found that each crop responded differently to fertilization within crop sequences and years, but that crops on continuous cropping systems always responded more. In evaluating the effect of fertilizers on crop yield in drier regions, Viets (1966) concluded that, "most evidence shows that fertilizers can be used in the drier areas to correct nutrient deficiencies and to make more efficient use of a limited supply of water. Yield reductions from use of fertilizers are rare". Viets also stated that there can be "no doubt that fertilizers on dryland often stimulate growth and increase the water use rate. The significance of this depends on the balance between need and supply at critical phases of growth and whether rains come at the right time". To evaluate these effects on yield, he suggested that "the best approach is to examine the results of a large number of field trials conducted over a number of years in the geographical area of interest". This is precisely the kind of evaluation provided by a well-adapted crop growth model with inputs of long-term weather data and soils information. CONCLUSIONS

In the semiarid Great Plains, management practices must be fine tuned for most efficient use of the limited water supply. Variations in softs and in weather require management practices to be site, soil, and crop specific. Because of the m a n y possible combinations, the specific recommendations can best be quantified through a mechanistic modeling approach followed by estimations of yield frequencies and confidence bounds. The general computer simulation methods used in this study can be applied to a range of models, site locations, and management techniques.

110

The steps involved can be summarized as follows: (1) Make a series of seasonal, annual, or crop rotation sequences over the desired range of precipitation (or climate) and management techniques, or parameter values representing those techniques. The objective is to only cover the ranges in detail sufficient to define the important trends. (2) Using the simulated values in (1) above, develop functional relationships for yield vs precipitation (site specific). Use regression or other curve fitting techniques. (3) Use a stochastic climate generator or various time sequences from the historical relationships in item (2) above to develop the yield frequency relationship for each management practice. This approach has the distinct advantage that long-term yield frequency information can be obtained from the application of a variety of relatively complex and comprehensive computer models without expending large amounts of resources. Skill is needed in selecting the data ranges and densities which are used. Some situations may require a continuous or semicontinuous simulation over an extended time period. The decision concerning the type of analysis required is critical. The examples presented in this paper illustrate types of analyses which do not require continuous simulations.

REFERENCES Aase, J.K., Siddoway, F.H. and Black, A.L., 1976. Perennial grass barriers for wind erosion control, snow management and crop production. In: R.W. Tinus (Editor), Shelterbelts on the Great Plains. Proc. Syrup., 20--22 April 1976, Denver, CO. Great Plains Agric. Counc. Publ. 78, pp. 69--78. Allmaras, R.R., 1967. Soft water storage as affected by infiltration and evaporation in relation to tillage-induced soil structure. In: Conf. Proc. Tillage for Greater Crop Production, 12--17 December 1967, Detroit, MI. PROC-168, American Society of Agricultural Engineers, St. Joseph, MI, pp. 37--43. Allmaras, R.R., 1983. Soil conservation: using climate, soils, topography, and adapted crops information to select conservation practices. In: H.E. Dregne and W.O. Willis (Editors), Dryland Agriculture in North America. Am. Soc. Agron. Monogr., 23: 139-153. Amemiya, M., 1968. Tillage-soil water relations of corn as influenced by weather. Agron. J., 60 : 534--537. Bauer, A.Young, R.A. and Ozbun, J.L., 1965. Effects of moisture and fertilizer on yields of spring wheat and barley. Agron. J., 57: 354--356. Black, A.L. and Siddoway, F.H., 1971. Tall wheatgrass barriers for soil erosion control and water conservation. J. Soil Water Conserv., 26: 107--111. Black, A.L. and Siddoway, F.H., 1976. Dryland cropping sequences within a tall wheatgrass barrier system. J. Soil Water Conserv., 31: 101--105. Black, A.L., Soddoway, F.H. and Brown, P.L., 1974. Summer fallow in the Northern Great Plains (winter wheat). In: Summer Fallow in the Western United States., Conserv. Res. Rep. 17, Agricultural Research Service, U.S. Department of Agriculture, Washington, DC, pp. 36--50.

111 Black, C.A., 1966. Crop yields in relation to water supply and soil fertility. In: W.H. Pierre, D. Kirkham, J. Pesek and R. Shaw (Editors), Plant Environment and Efficient Water Use. American Society of Agronomy/Soil Science Society of America, Madison, WI, pp. 177--206. Burnett, E. and Hauser, V.L., 1967. Deep tillage and soil-plant-water relationships. In: Conf. Proc. Tillage for Greater Crop Production, 11--12 December 1967, Detroit, MI. PROC-168, American Society of Agricultural Engineers, St. Joseph, MI, pp. 47--57. BurweU, R.E. and Larson, W.E., 1969. Infiltration as influenced by tillage-induced random roughness and pore space. Soil Sci. Soc. Am. Proc., 33 : 449--452. Campbell, C.S., 1977. An Introduction to Environmental Biophysics. Springer-Verlag, New York, NY, 159 pp. CIA, 1974. USSR Agricultural Atlas. Central Intelligence Agency, U.S. Government Printing Office, Washington, DC, 59 pp. Cochran, V.L., Warner, R.L. and Papendick, R.I., 1978. Effect of N depth and application rate on yield, protein content, and quality of winter wheat. Agron. J., 70: 964-968. Compton, L.L., 1942. Moisture conservation practices and the relationship of conserved water to crop yields. Soil Sci. Soc. Am. Proc., 7: 368--373. De Jong, R. and Best, K.F., 1979. The effect of soil water potential, temperature, and seedling depth on seedling emergence of wheat. Can. J. Soil Sci., 59: 259--264. Dutt, G.R., Shaffer, M.J. and Moore, W.J., 1972. Computer simulation model of dynamic bio-physicochemical processes in soils. Univ. Ariz. Agric. Exp. Stn. Tech. Bull. 196, 101 pp. Greb, B.W., 1979a. Technology and wheat yields in the Central Great Plains: Commercial advances. J. Soil Water Conserv., 34: 269--273. Greb, B.W., 1979b. Reducing drought effects on croplands in the West-Central Great Plains. U.S. Dep. Agric. Inf. Bull. 420, 31 pp. Greb, B.W., Smika, D.E. and Black, A.L., 1970. Water conservation with stubble mulch fallow. J. Soil Water Conserv., 25: 58---62. Greb, B.W., Smika, D.E., Woodruff, N.P. and Whitfield, C.J., 1974. Summer fallow in the Central Great Plains. In: Summer Fallow in the Western United States. Conserv. Res. Rep. 17, Agricultural Research Service, U.S. Department of Agriculture Washington, DC, pp. 51--85. Greb, B.W., Smika, D.E. and Welsh, J.R., 1979. Technology and wheat yields in the Central Great Plains: Experiment Station advances. J. Soil Water Conserv., 34: 264-268. Gul, A. and Allan, R.E., 1976. Stand establishment of wheat lines under different levels of water potential. Crop Sci., 16: 661--615. Hadas, A. and Russo, D., 1974. Water uptake by seeds as affected by water stress,capillary conductivity and seed-soil water contact. If. Analysis of experimental data. Agron. J., 66: 647--652. Hanks, R.J. and Hill, R.W., 1980. Modeling Crop Responses to Irrigation in Relation to Soils, Climate, and Salinity. Pergamon Press, Oxford, 66 pp. Hart, J., 1964. Wide choice in wheat and barley planting depth. Queensl. Agric. J., 90: 137--138, Abstr. Biol. Abstr. 47(1), No. 4054. Herkelrath, W.N., Miller, E.E. and Gardner, W.R., 1977. Water uptake by plants: If. The root contact model. Soil Sci. Soc. Am. J., 41: 1039--1043. Hillel, D., 1972, Soil moisture and seed germination. In: T.T. Kozlowski (Editor), Water Deficits and Plant Growth, Vol. III. Academic Press, N e w York, NY, pp. 65--89. Hillel, D. and Hadas, A., 1972. Isothermal drying of structurally layered soil columns. Soil Sci., 113: 30--35.

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